From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755335Ab2HOBI3 (ORCPT ); Tue, 14 Aug 2012 21:08:29 -0400 Received: from out02.mta.xmission.com ([166.70.13.232]:37095 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751444Ab2HOBIZ (ORCPT ); Tue, 14 Aug 2012 21:08:25 -0400 From: ebiederm@xmission.com (Eric W. Biederman) To: Sasha Levin Cc: torvalds@linux-foundation.org, tj@kernel.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, paul.gortmaker@windriver.com, davem@davemloft.net, rostedt@goodmis.org, mingo@elte.hu, aarcange@redhat.com, ericvh@gmail.com, netdev@vger.kernel.org, josh@joshtriplett.org, eric.dumazet@gmail.com, mathieu.desnoyers@efficios.com, axboe@kernel.dk, agk@redhat.com, dm-devel@redhat.com, neilb@suse.de, ccaulfie@redhat.com, teigland@redhat.com, Trond.Myklebust@netapp.com, bfields@fieldses.org, fweisbec@gmail.com, jesse@nicira.com, venkat.x.venkatsubra@oracle.com, ejt@redhat.com, snitzer@redhat.com, edumazet@google.com, linux-nfs@vger.kernel.org, dev@openvswitch.org, rds-devel@oss.oracle.com, lw@cn.fujitsu.com References: <1344961490-4068-1-git-send-email-levinsasha928@gmail.com> <1344961490-4068-3-git-send-email-levinsasha928@gmail.com> <87txw5hw0s.fsf@xmission.com> <502AF184.4010907@gmail.com> Date: Tue, 14 Aug 2012 18:08:09 -0700 In-Reply-To: <502AF184.4010907@gmail.com> (Sasha Levin's message of "Wed, 15 Aug 2012 02:47:00 +0200") Message-ID: <87393phshy.fsf@xmission.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=;;;mid=;;;hst=in02.mta.xmission.com;;;ip=98.207.153.68;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/rAtJ926uK+UsFkv5L6lexpbCQUydxfqo= X-SA-Exim-Connect-IP: 98.207.153.68 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.0 T_TM2_M_HEADER_IN_MSG BODY: T_TM2_M_HEADER_IN_MSG * -3.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% * [score: 0.0001] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa02 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 T_TooManySym_01 4+ unique symbols in subject X-Spam-DCC: XMission; sa02 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: ;Sasha Levin X-Spam-Relay-Country: Subject: Re: [PATCH 02/16] user_ns: use new hashtable implementation X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Fri, 06 Aug 2010 16:31:04 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Sasha Levin writes: > On 08/15/2012 01:52 AM, Eric W. Biederman wrote: >> Sasha Levin writes: >> >>> Switch user_ns to use the new hashtable implementation. This reduces the amount of >>> generic unrelated code in user_ns. >> >> Two concerns here. >> 1) When adding a new entry you recompute the hash where previously that >> was not done. I believe that will slow down adding of new entries. > > I figured that the price for the extra hashing isn't significant since hash_32 > is just a multiplication and a shift. > > I'll modify the code to calculate the key just once. Honestly I don't know either way, but it seemed a shame to give up a common and trivial optimization. >> 2) Using hash_32 for uids is an interesting choice. hash_32 discards >> the low bits. Last I checked for uids the low bits were the bits >> that were most likely to be different and had the most entropy. >> >> I'm not certain how multiplying by the GOLDEN_RATION_PRIME_32 will >> affect things but I would be surprised if it shifted all of the >> randomness from the low bits to the high bits. > > "Is hash_* good enough for our purpose?" - I was actually surprised that no one > raised that question during the RFC and assumed it was because everybody agreed > that it's indeed good enough. > > I can offer the following: I'll write a small module that will hash 1...10000 > into a hashtable which uses 7 bits (just like user_ns) and post the distribution > we'll get. That won't hurt. I think 1-100 then 1000-1100 may actually be more representative. Not that I would mind seeing the larger range. Especially since I am in the process of encouraging the use of more uids. > If the results of the above will be satisfactory we can avoid the discussion > about which hash function we should really be using. If not, I guess now is a > good time for that :) Yes. A small emperical test sounds good. Eric